This invention relates to electrical measurements in general and more particularly to an improved circuit arrangement for storing the phase of an a-c voltage.
Circuit arrangements for storing the phase of an a-c voltage are employed, among other things, in equipment for the protection of electric power transmission lines. For instance, such storage is used where it is necessary to reliably determine the direction of the location of a short circuit as seen from the place of installation of the equipment. Two measurement quantities are required for this direction determination, namely, for instance, current and voltage in the event of a short circuit at the place of installation of the equipment, so that conclusions as to the direction of the short circuit can be drawn from their phase difference. The determination of the direction then depends on the fact that a short circuit voltage is also present in the event of a short circuit at the place of installation of the equipment.
This condition is not always fulfilled, so that one endeavors to utilize, in a polyphase power transmission, voltages of those phases which are not involved in the short circuit. However, since in certain fault cases, such voltages "removed from the short circuit" are not present either, circuit arrangements for storing the phase of the short circuit voltage are used. Known circuit arrangements of this type (Siemens Catalog R2, 1978 "Conventional Protective Apparatus and Relays," page 5/45) consist of tuned circuits which maintain the phase prior to the occurrence of the short circuit for a time long enough, during the decay of the voltage, to determine the direction, even if the voltage at the power transmission system to be monitored breaks down completely.
The use of circuit arrangements consisting of tuned circuits for storing the phase of an a-c voltage presents difficulties especially if a reversal of the polarity of the voltage must be expected in the event of a short circuit. This applies, for instances, to power transmission lines with series capacitors if the short circuit occurs behind a capacitor, since in this case the tuned circuit must be decoupled from the voltage as fast as possible so that the voltage does not pull it into a false synchronism.
It is thus an object of the present invention to provide a circuit arrangement for storing the phase of an a-c voltage which ensures that the original phase is stored, even if the a-c voltage disappears and even if its phase is changed suddenly.